1. Technical Field
The present invention relates to a method of forming spring washer (seat) blind-holes of a piston for an automobile transmission, wherein a plurality of spring washer blind-holes on the back of an end plate of the piston for the automobile transmission are formed efficiently, keeping endurance of pins for piercing the spring washer blind-holes, and precisely by cold forging.
2. Background Art
At the moment, it seems that a method of forming spring washer blind-holes of a piston for an automobile transmission through forging technology has not been proposed yet.
For example, it is shown in Patent Document 1 that spring washer blind-holes (they are simply referred to as recesses in the document) are disposed on the inner surface of the end plate of the piston in a multiple-disc clutch. However, there is no comment upon the method of forming them.
With regard to the method of manufacturing oil hydraulic machine parts in Patent Document 2, a combined method of die casting and plastic working is introduced as a production process of pistons for an automobile transmission. No examples of a piston having spring washer blind-holes on its end plate are provided in the document, and naturally no description on any spring washer blind-holes is given there.
About formation of spring washer blind-holes in another industrial field, there are some proposals such as Patent Documents 3, 4, and 5. However, all of the proposed methods of forming spring washer blind-holes are entirely different than that of the present invention in material, size, etc. of the workpieces, and even in the manufacturing methods. The proposed methods do not fall under forging technology, and have nothing to do with the present invention.
As far as the applicant is concerned, it seems that spring washer blind-holes of a piston for an automobile transmission are, at present, generally formed by drilling. When spring washer blind-holes are formed by drilling, the bottom end of the holes formed will be in the shape of a steeple-crowned cap corresponding to that of the tip of a drill, that is to say, the nearer to the center of the hole the gradually deeper the bottom end surface of the hole is. Since a hole with such a shape as above cannot stably hold the end of the spring, subsequent retouch machining by an endmill will necessarily be performed to flatten the bottom end. When spring seat blind-holes are formed by drilling, the drilling operations should naturally be repeated as many times as the number of spring seat blind-holes to be formed. The operations above are very inefficient, and moreover, subsequent endmilling operations in the same number as the drilling operations are also necessary and more inefficient in cost.
Accordingly, a fixing seat has been proposed in Patent Document 6. The fixing seat is a member comprising a lower portion in the shape of a cone or a truncated cone and an upper portion formed into a flat plate. Inserting and disposing the fixing seat under the drilled holes to provide flat bottom ends to the holes, the fixture is intended to be used as an appropriate spring seat and the like. The document suggests that the holes applied to spring seat blind-holes are impossible to be formed only by drilling, and proposes a means to solve the problem without using any endmilling operation. However, the proposal has not solved such inefficiency that extra parts should be employed and that drilling operations should be repeated as many times as the number of spring seat blind-holes to be required.
Besides, in the case of the present invention, the thickness of the bottom end of the spring seat blind-holes must come thin due to the formation of spring seat blind-holes on the back of end plates of pistons for automobile transmissions. When trying to form the holes by drilling, and where the spring seat blind-holes are bored on the end plates of pistons for automobile transmissions, shear stress due to drilling will occur at the tip of a drill, and it is liable to result in occurrence of strain cavities or cracks on the bottom end because of the thin bottom end.